One of the ways to create a vignette effect in Photoshop CC is with the lens correction filter; it is quick and easy to apply and produces consistent results. Another method using masked layers is flexible and can produce a near-endless assortment of results.
Using the Lens Correction Filter
Open an image in Photoshop, click "Filter" and select "Lens Correction."
Select the "Custom" tab.
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Ensure the "Preview" box under the photo is checked and drag the two sliders in the Vignette section to adjust the vignetting. Dragging the "Amount" slider away from the middle point darkens or lightens the corners of the image, depending on the direction in which you slide it. Dragging the "Midpoint" slider to the left reduces the area of the image that is left untouched, while dragging it to the right increases it. When you have a result you like, click "OK" at the top.
Using Masked Layers
Open an image in Photoshop. Click and hold the "Rectangle Tool" button in the Tools bar and select "Ellipse Tool."
Ensure the Pick Tool Mode drop-down menu in the Options bar is set to "Shape" and draw an ellipse on the image, covering the area you want to leave unaffected by the vignetting. Using an ellipse shape instead of drawing an elliptical selection with the Marquee tool makes the shape easier to alter in terms of shape, size and positioning.
Press "Ctrl-T" to enter free transform mode. Drag the handles to resize and reshape the ellipse and drag the ellipse around the canvas to reposition it. When you're satisfied, press "Enter." If you want to center the ellipse on the canvas, press "V" to select the Move tool, press "Ctrl-A" to select the entire canvas, click the "Align Horizontal Centers" and "Align Vertical Centers" buttons on the Options bar to reposition the ellipse and then press "Ctrl-D" to deselect the canvas.
Hold "Ctrl" and click the thumbnail image for the ellipse in the Layers pane. This creates a selection precisely matching the ellipse.
Click the eye button to the left of the ellipse shape to hide it. For the time being, you no longer need it; however, you will need it in the future if you want to recreate the selection without starting from scratch.
Click "Select" and select "Inverse" to invert the selection. This selects the area that will be occupied by the vignetting effect instead of the area that will be left untouched.
Click and hold the "New Adjustment or Fill Layer" button in the Layers pane to bring up a list of layer types.
Click the menu entry corresponding to the layer type you want to create. Which layer you create at this point depends on the kind of effect you want. For a standard vignette, try a Brightness/Contrast layer or select a Levels layer for tighter control. Using a Solid Color or a Pattern layer and experimenting with the blending modes produces unorthodox but visually interesting effects, as does using a Vibrance layer. Whichever layer you create, it is automatically masked to affect only the area that is selected.
Click the thumbnail for the layer mask belonging to the newly created layer in the Layers pane to bring up the Properties pane.
Drag the "Feather" slider to the right to make the edges of the mask fuzzier, providing a smoother selection between the untouched area and the vignetting effect.
Adjust the settings for the layer, as well as its blending mode and opacity, until you're satisfied with your image.